Frictional Pivots for Gravitational Alignment

ABSTRACT

A frictional pivot  100  for use in a device measuring gravitational alignment is provided. The frictional pivot  100  comprises a gravity-responsive directional means  200  for indicating a datum direction of alignment with gravity; frictional pivoting means  300, 400  for allowing the gravity-responsive means coarsely to align with gravity; vibration means  303, 403  for vibrating one or more elements of the pivoting means; and portable power means  303   a  for powering the vibration means  303, 403.

BACKGROUND OF THE INVENTION

This invention relates to devices containing frictional pivots ingeneral, and more particularly but not necessarily to devices such aslaser levels, which rely on frictional pivots for gravitationalalignment of pendulous components about an axis.

The accuracy of such a device depends upon the accuracy with which thependulous components align with gravity. Any misalignment leads toinaccuracy in the device. Friction in the point or points about whichthe pendulous components pivot is the chief cause of this misalignment.Conventionally, this problem is addressed by reducing friction in thepivot by employing such means as lubrication or precision rollerbearings.

Unfortunately, this conventional solution generates further problems,one of which is that as friction is reduced in the pivot, so thependulous components take longer to stop oscillating and come to rest.As accurate readings cannot be taken until the pendulous components cometo rest, the lower the friction of the bearing, the longer the user mustwait before taking a reading and therefore the more inconvenient it isto use the device. The conventional solution to this problem is to applydamping means to reduce the oscillations.

A further problem with very low-friction pivots is that the pendulouscomponents are easily disturbed by stray environmental influences,setting up further oscillations of the pendulous components, incurringfurther delays in readings and inconvenience to the user.

Thus it can be seen that the conventional solution to the problem ofinaccuracy caused by misalignment leads to expensive and complicatedsolutions which generate further problems necessitating more expense andcomplication in their solution.

SUMMARY OF THE INVENTION

The principal object of this invention is to address all of theseproblems simply and economically without generating further problems.

To see how this may be done, first consider the forces at work in asimple rigid pendulum, having a weight at one end and a pivot at theother. When released from a position in which the weight is notgravitationally aligned, the gravitational restoring force causes adownward swing of the pendulum. The pendulum passes through the point ofgravitational alignment and continues on an upward swing, slowing downas friction in the pivot and gravity act upon the pendulum, reducing itsvelocity until the angular momentum is zero and the pendulum stops, verybriefly, at the top of its swing. If the gravitational restoring forceis greater than the static friction in the pivot, then the pendulum willrepeat this cycle. If not, then the pendulum will remain motionless. Thereason for this lies in the fact that, when the pendulum is moving, itsmovement is resisted by the dynamic or rolling friction in the pivot. Atthe end of the pendulum's swing, before it reverses direction, it comesto a halt, and, at that point the resistance to any further movement isdue not to dynamic friction, but to static friction. Static friction isconsiderably higher than dynamic friction at this point. The pendulumwill always stop at the end of a cycle and that point will always bebeyond the point of gravitational alignment. The distance between thesetwo points will be proportional to the friction in the pivot and willrepresent the ultimate accuracy of the device relying upon thisalignment. With a high-friction pivot this accuracy will be poor. With alow-friction pivot, the accuracy of alignment will be better, but thelower friction in the bearing will mean an increased number ofoscillations of the pendulum leading to a delay in the pendulum comingto rest.

The present invention addresses these problems simply and inexpensivelyby employing a pivot which is designed to be deliberately andcontrollably frictional.

According to the present invention, a frictional pivot comprisesgravity-responsive directional means for indicating a datum direction ofalignment with gravity, frictional pivoting means for allowing thegravity-responsive means coarsely to align with gravity, vibration meansfor vibrating one or more elements of the pivoting means, and portablepower means for powering the vibrating means.

The length and frequency of occurrence of vibration produced by thevibrating means might be controlled by manual switches or electronictiming circuitry.

Pendulous movement is resisted by the frictional force in the pivot,thus preventing free oscillations of the pendulous components, but thepivot is not so frictional as to inhibit the pendulous components fromcoarsely aligning with gravity.

Vibration is applied to one or more members of the pivot. This vibrationcauses one element of the pivot to move fractionally relative to theother. This converts the static friction into dynamic friction. Thus,for the duration of the pulse of vibration, further movement is enabledand the pendulous components, under the influence of the gravitationalrestoring force, move closer to gravitational alignment. So that themomentum of the pendulous components does not cause them to move pastthe point of gravitational alignment, the pulses of vibration should beshorter than a quarter of the period of the pendulum. The vibration ofthe pendulum therefore ceases around the position of gravitationalalignment and the comparatively high static friction in the un-vibratedpivot will reduce the extent of the motion beyond the position ofgravitational alignment, thus ensuring that the pivot comes to restclose to a position of gravitational alignment. In addition, thecomparatively high static friction of the pivot renders the pivotrelatively immune from environmental disturbances which would adverselyaffect instruments employing a low-friction bearing.

Experiments have shown that a 10 gm mass at 40 mm radius from the axisof rotation may be displaced by plus or minus 1.623 degrees fromgravitational alignment in an un-vibrated pivot. Using the same pivot,after three seconds of vibration this displacement is reduced to plus orminus 0.00955 degrees from gravitational alignment. With pulsedvibration as described above the settling time may be further reduced.

It will thus be appreciated that the present invention can enhance theaccuracy and convenience of such instruments utilising gravitationalalignment while at the same time reducing the expense of suchinstruments by allowing the replacement of expensive precision rollerbearings by inexpensive pivots.

A first embodiment of the invention has a reference point at one end ofa weighted pendulous arm orthogonally attached to a pivotable shaft.Conical shaft ends are located in conical depressions in two opposingplates of a flexible material which are held apart at a predetermineddistance by being rigidly attached to a case. A region of the platesprojects beyond the case and is unsupported. In this region is locatedthe conical depression and the vibration means. A small, low-voltageelectric motor provides an inexpensive vibration means with an axiallyattached eccentric weight. Motors of this type are used in mobile phonesand pagers.

A second embodiment of the invention includes an eccentrically weightedcylindrical housing frictionally attached about a common axis to anothercylindrical housing, the second housing containing a laser projectingmeans. In this embodiment, a reference point indicating gravitationalalignment is a mark on the circumference of the weighted housing, withother marks spaced at regular angular intervals on the circumference ofthe second housing indicating the angular displacement of the laserprojecting means away from the gravitational vertical.

Another embodiment of the invention might place the vibrating meanswithin one or both of the cylindrical housings.

In a further embodiment one or both of the two opposing plates arereplaced by slugs of material with conical depressions. These slugs areaxially movable relative to the pivotable shaft and are held against theends of the shaft by leaf or helical springs to provide the frictionalpre-loading of the pivot.

Although the following description sets out a number of distinctexamples of the present invention it will be evident to one skilled inthe art that the various features could be combined to form furthersimilar examples.

BRIEF DESCRIPTION OF THE DRAWINGS

Frictional pivots, in accordance with the present invention, will now bedescribed in more detail, by way of example only, with reference to theaccompanying drawings, in which:

FIG. 1 is a perspective view of a first embodiment of a frictional pivotaccording to the present invention;

FIGS. 2, 3 and 4 are side, front and plan views of FIG. 1, respectively;and

FIG. 5 is a perspective view of a second embodiment of a frictionalpivot according to the present invention.

FIGS. 6 and 7 are plan and rear views of FIG. 5, respectively.

DETAILED DESCRIPTION OF THE INVENTION

In all of the drawings, switching/timing means and portable power meanshave been omitted for clarity.

FIGS. 1 to 4 show a frictional pivot 100 comprising a pendulous assembly200 connected via axial shaft 500 to pivot halves 300 and 400 which arerigidly attached to case 600.

The pendulous assembly 200 includes a pendulum 201, weight 202 andreference point 203. Pivot half 300 is a plate 301 with a conicaldepression 302 and vibrating means 303. Plate 301 is rigidly attached tocase 600 in such a way that an elongate portion comprising the conicaldepression 302 and the vibrating means 303 is able to move axially withreference to shaft 500. This movement is governed by the gauge andspringiness of plate 301. Similarly pivot half 400 comprises plate 401with a conical depression 402 and vibrating means 403. Axial shaft 500has conical ends 501 and 502 which locate in conical depressions 302 and402. The relative angles of shaft end and depression may be such thatonly the points of the shaft bear on the conical depressions. The forcewith which axial shaft 500 is held between conical depressions 302 and402 may be varied according to the flexibility of plates 301 and 401.Plates 301 and 401 may be angled inwards to increase this force, therebyincreasing the friction between plates 301 and 401 and shaft 500.

Vibrating means 303 and 403 may be of an electromechanical,magnetostrictive or piezoelectric nature. The switching/timing controls(not shown) are existing and well known and the common 555 timer type ofintegrated circuit such as NE555N manufactured by FairchildSemiconductor Corporation, or the NE555P manufactured by TexasInstruments, or equivalent may provide pulses from seconds tomilliseconds. Thus it will be seen that two such integrated circuits maybe employed, one to control the duration and the other to control thefrequency of occurrence of the vibration pulses.

The pendulous assembly 200, pivot halves 300 and 400 and case 600,together, form a tuned mechanical assembly, the design and selection ofthe components being inter-dependent. The mass of pendulous assembly 200may be varied, depending on the application in which the presentinvention is employed. The rigidity of plates 301 and 401 and thestrength of vibration applied by the vibration means 303 and 403 must bechosen to accommodate the mass of the pendulous assembly.

FIGS. 5 to 7 show an example of a laser light referencing toolincorporating a frictional pivot 100 a comprising pendulous assembly 200a connected via axial shafts 500 a and 500 b to pivot halves 300 a and400 a which are rigidly attached to a case.

Pendulous assembly 200 a includes a first cylindrical housing 201 awhich contains laser projecting means emitting a beam towards laseraperture 700 a. The first housing 201 a also has angular reference marks203 a about its circumference. The first cylindrical housing 201 a isfrictionally attached about a common axis by shafts 500 a and 500 b to asecond cylindrical housing 201 b, which is eccentrically weighted andhas a reference point 203 b to indicate gravitational alignment.

Pivot half 300 a is a thin strip of phosphor bronze with a conicaldepression 302 a, rigidly attached to the case by fasteners 601 b.

Pivot half 400 a is also a thin strip of phosphor bronze. It has aconical depression 402 a and is rigidly attached to the case byfasteners 601 a. At the free end of pivot half 400 a, a 1.5 volt motor303 a with axially attached eccentric weight 303 b is secured.Electrical power is supplied to the laser light referencing tool and/orthe vibration means 303, 403 through the pivot halves 300 a and 400 a.

The shafts 500 a and 500 b have conical ends 501 a and 501 b,respectively which locate in conical depressions 302 a and 402 a inpivot halves 301 a and 401 a.

Frictional pivots in accordance with the present invention areparticularly suitable for use in laser light referencing tools of thekind disclosed in European patent publication EP-A-1012538 (WO98/11407). When used in place of plain pivots, substantial gains inaccuracy are achieved. When used in place of roller or ball bearings,great cost savings are realised without any sacrifice of accuracy andwithout the need for further damping means. The tool is also relativelyimmune to external environmental disturbances. Thus it will be seen thatinexpensive, easy to use and accurate level, plumb and angularmeasurements are now possible.

1. A frictional pivot comprising: gravity-responsive directional meansfor indicating a datum direction of alignment with gravity; frictionalpivoting means for allowing the gravity-responsive means coarsely toalign with gravity; vibration means for vibrating one or more elementsof the pivoting means; and portable power means for powering thevibration means.
 2. A frictional pivot according to claim 1, wherein thelength and frequency of occurrence of vibration produced by thevibrating means are controlled by manual switches or electronic timingcircuitry.
 3. A frictional pivot according to claim 1 or 2, wherein thegravity responsive directional means is a weighted pendulous armorthogonally attached to a pivotable shaft.
 4. A frictional pivotaccording to claim 1 or 2, wherein the gravity responsive directionalmeans is an eccentrically weighted element orthogonally attached to apivotable shaft
 5. A frictional pivot according to claim 3 or 4, whereinthe frictional pivoting means are two opposing plates of a flexiblematerial which are held apart at a predetermined distance by beingrigidly attached to a case, and conical ends of the shaft are located inconical depressions in the two opposing plates.
 6. A frictional pivotaccording to any one of the preceding claims, wherein the vibrationmeans is a low-voltage electric motor with an axially attached eccentricweight.
 7. A frictional pivot according to claim 5, wherein one or bothof the two opposing plates are slugs of material with conicaldepressions and the slugs are axially movable relative to the pivotableshaft and held against the ends of the shaft by leaf or helical springs.8. A laser referencing tool having a frictional pivot according to anyone of claims 1 to
 7. 9. A laser referencing tool according to claim 7when dependent on claim 4, wherein the laser referencing tool isintegrally formed with the gravity-responsive directional means.
 10. Alaser referencing tool according to claim 8 or 9, further comprising aneccentrically weighted cylindrical housing frictionally attached about acommon axis to another cylindrical housing, the second housingcontaining laser projecting means.
 11. A laser referencing toolaccording to claim 10, wherein a reference point indicatinggravitational alignment is a mark on the circumference of the weightedhousing, with other marks spaced at regular angular intervals on thecircumference of the second housing indicating the angular displacementof the laser projecting means away from the gravitational vertical. 12.A laser referencing tool according to claim 10 or 11, wherein thevibrating means is within one or both of the cylindrical housing.